An exposure apparatus for exposing a mask pattern onto a photosensitive-agent applied substrate have conventionally been used to manufacture semiconductor devices and liquid crystal panels. Since finer processing of a pattern is demanded for improved integrations of devices, exposure apparatuses are improved so as to resolve fine patterns.
The following Rayleigh equation (1) defines resolution R of a projection optical system in an exposure apparatus, which is an index of a size of a resolvable pattern:R=k1(λ/NA)  (1)where λ is an exposure wavelength, NA is a numerical aperture of the projection optical system at its image side, and k1 is a constant determined by a development process and others, which usually is approximately 0.5.
As understood from Equation (1), the resolving power of the optical system in the exposure apparatus becomes higher as the exposure wavelength is shorter and the image-side NA of the projection optical system is greater.
Therefore, following the mercury lamp i-line (with approximately 365 nm in wavelength), a KrF excimer laser (with approximately 248 nm in wavelength) and an ArF excimer laser (with approximately 193 nm in wavelength) have been developed, and more recently an F2 excimer laser (with approximately 157 nm in wavelength) is reduced to practice. However, a selection of the exposure light having a shorter wavelength makes it difficult to meet material requirements with respect to transmittance, uniformity and durability, etc., causing an increasing cost of the apparatus.
An exposure apparatus having a projection optical system with a NA of 0.85 is commercially available, and a projection optical system with a NA of 0.9 or greater is researched and developed. Such a high-NA exposure apparatus has difficulties in maintaining good imaging performance with little aberration over a large area, and thus utilizes a scanning exposure system that synchronizes the mask with a substrate during exposure.
However, a conventional design cannot make the NA greater than 1 in principle due to a gas layer having a refractive index of about 1 between the projection optical system and the substrate.
On the other hand, an immersion method is proposed as means for improving the resolving power by equivalently shortening the exposure wavelength. It is a method used for the projection exposure, which fills liquid in a space between the final surface of the projection optical system and the substrate, instead of filling this space with air as in the prior art. The projection exposure apparatus uses, as the immersion method, a method for immersing the final surface of the projection optical system and the entire substrate in the liquid tank (see, for example, Japanese Patent Application, Publication No. 6-124873), and a local fill method that flows the fluid only in the space between the projection optical system and the substrate (see, for example, International Publication No. WO99/49504 pamphlet).
The immersion method has an advantage in that the equivalent exposure wavelength has a wavelength of a light source times 1/n, where n is a refractive index of the used liquid. This means that the resolving power enhances by 1/n times the conventional resolving power, even when the light source having the same wavelength is used.
For example, when the light source has a wavelength of 193 nm and the fluid is the water, the refractive index is about 1.44. Therefore, use of the immersion method can improve the resolving power by 1/1.44 times the conventional method.
The most common fluid used for the immersion method is water. The water has a good transmittance relative to the ultraviolet light down to about 190 nm. In addition, advantageously, a large amount of water is used in the semiconductor manufacturing process, and the water gets along with the wafer and photosensitive agent.
It is important to reduce the influence of the air gas bubbles to the exposure in the immersion exposure apparatus. These gas bubbles that enter the exposure area between the final surface of the projection optical system and the substrate scatter the exposure light. Therefore, the transferred pattern's critical dimension varies beyond the permissible range, causing insulations and short circuits contrary to the design intent in the worst case. Degassing of the fluid is the most effective, known method to prevent the influence of the gas bubbles to the exposure. Since the gas bubbles are unlikely to occur or the generated gas bubbles extinguish in a short time period in the degassed fluid, the influence of the gas bubbles to the exposure is prevented.
However, the water widely used for the immersion method increases the resistivity, when degassed, and is likely to generate static electricity disadvantageously. For example, the pure water (i.e., the water containing few impurities) used for the semiconductor manufacturing process reaches the resistivity of 18 MΩ·cm after degas. The substrate surface is electrically insulated since the photosensitive agent is applied on it. Therefore, as the stage moves the substrate, the static electricity is generated on the substrate surface and makes the device on the substrate defective.